Known in the art is an internal combustion engine arranging a catalyst suitable for reducing NOx in exhaust gas by ammonia in the presence of excess oxygen in an engine exhaust passage, feeding an aqueous urea solution to this catalyst, storing part of the urea fed to the catalyst in the catalyst, and using the ammonia generated from the urea stored in the catalyst to reduce the NOx in the exhaust gas (see Japanese Patent Publication No. 3685063).
However, the exhaust gas sometimes contains formaldehyde. When this formaldehyde reacts with the urea stored in the catalyst, urea resin is generated. Therefore, when the exhaust gas contains a large amount of formaldehyde, if feeding urea to the catalyst, not only can the urea not be effectively utilized for NOx reduction, but also a large amount of urea resin is formed on the catalyst and therefore the active sites of the catalyst are liable to be covered and the catalyst function to decrease or the catalyst to clog. If the cell pitch of the catalyst is made larger to suppress this clogging, the catalyst carried amount is liable to decrease or the contact frequency of the catalyst is liable to decrease. In any case, the exhaust purification performance of the catalyst is liable to decrease. Even if arranging in the exhaust passage upstream of the catalyst a hydrolysis catalyst for promoting hydrolysis of urea and a dispersing plate for dispersing the urea, the hydrolysis catalyst or dispersing plate is also liable to become clogged.
On the one hand, if the HC and CO in the exhaust gas and the urea or ammonia etc. react, sometimes cyanogen compounds will be generated. Therefore, when the exhaust gas contains a large amount of HC and CO, if feeding the urea to the catalyst, not only can't the urea or ammonia be effectively utilized for NOx reduction, but a large amount of cyanogen compounds is liable to be generated.